Click on details > press vote to show you support it so that they will implement it.

Also if you want tell them why would you like it to be implemented like:

- Free up memory (RAM)- Clear your browser cache if that option is set- Remove cookies if that option is set- Bugs (in Nightly) might require a restart to investigate properly (especially with the "tabs" API)- Reload things that have changed on disk (configuration files)- Start fresh without all the previous tabs as a way to clear the mind and focus.